The present invention generally relates to polymeric structures, and more particularly to actuators and methods utilizing electrical properties of polymer materials.
Components formed of polymeric materials are often critical to the performance of the products in which they are used, which encompasses various industries including medical, automotive, aviation, aerospace, appliances, and many others. A particular example is the catastrophic failure of a seal, which can be extremely detrimental to a device or machine in which it is installed. As used herein, a seal is a component capable of use in a static and/or dynamic system and is responsible for preventing leaks, maintaining system pressure, preventing contamination, or another similar function relating to the retention or separation of fluids (gases and liquids). Conventional seals are primarily composed of rubber materials due to their advantageous mechanical properties, including flexibility, high bulk modulus (resistance to change in volume under loads), resistance to cuts and tears, long fatigue life, ease of manufacturability, and low cost. When stress or strain energy is applied to a seal, the rubber material internally rearranges and can store energy or dissipate energy. When energy is stored the applied stress creates a corresponding strain resulting in a contact stress (sealing force). However, a loss of sealing force can occur as the polymer chains of the rubber material rearrange to reduce their internal energy over time, referred to as creep.
Isolated locations in which seals and other polymeric components are often installed complicate the ability to monitor their performance and structural failures using conventional electronic sensors. Nonetheless, methods and systems have been developed for the purpose of monitoring various polymer materials and structures for the purpose of detecting an impending failure. Notable examples include U.S. Pat. Nos. 7,555,936 and 7,752,904, which teach the incorporation of sensing elements embedded in polymeric materials.
In addition to detecting an impending catastrophic failure, the ability to reseal a sealing interface or at least reduce the amount of leakage at the interface would give operators additional time to order replacement seals, prepare to shut down the device, or take any other appropriate actions. As a nonlimiting example, o-rings (annular-shaped seals) are sized and shaped to be placed in a groove (gland) having prescribed dimensions to promote the sealing effect of the o-ring. O-ring grooves typically have square or rectangular-shaped cross-sections that, when an appropriate o-ring is placed into the groove, results in the o-ring being compressed to achieve a desired level of deformation. O-ring failure and resulting leakage can occur though a number of mechanisms, many of which can be traced to improper installation or mechanical damage. Another cause in seal failure is an inadequate compression setting that results in the development of an inadequate seal line. In addition to common seal configurations such as o-rings, the benefits of achieving a resealing capability would be desirable in a wide variety of seal types and shapes.
A group of polymeric materials known as electroactive polymers (EAP) have been considered for various applications due to their ability to convert electrical energy into mechanical motion through a process of deformation. Notable examples include actuators in which motion can be induced by the application of electrical energy to an EAP material. EAPs can be divided into two subcategories, ionic and electric EAPs. Dielectric EAP materials are a subclass of electric EAPs that are viscoelastic and exhibit properties similar to dielectric materials of capacitors when positioned between two conductive electrodes that apply a large voltage, for example, in a range of about 1000 to about 10,000 Vat low (micro-amp) current levels. In view of these conditions, the overall energy consumption or power to deform (actuate) a dielectric EAP material is low. When a sufficient electrical potential is applied to the electrodes, Coulomb forces cause electrostatic stresses to occur that cause the viscoelastic EAP material to reallocate its volume, forcing it to constrict in thickness and expand (strain) in the in-plane (length and width) directions. This deformation brings the oppositely charged electrodes into closer proximity relative to one another. When the electromagnetic field is removed, the EAP material substantially returns to its original state.
Various obstacles exist that have limited the manufacture and implementation of EAP materials in devices and machinery, including the generation of adequate forces and deformations, durability, and powering complications. Notable improvements have been achieved to increase deformations achievable with dielectric EAP materials by pre-straining the material prior to the application of the electrodes and through construction of multilayer systems. For example, certain types of dielectric EAP materials, examples being VHB 4905 and 4910 acrylic-based materials commercially available from 3M, can achieve pre-strains of greater than 200% that are capable of greatly increasing their actuation strains. Experimentation has been conducted with two-dimensional restricted flat sheets as well as multilayered and rolled configurations, resulting in increased achievable forces due to the summation of forces generated at each layer.